The fabrication of multilayered circuit boards is generally accomplished by depositing a sequence of dielectric and conductor layers. Metallized openings in the dielectric layers, commonly referred to as vias, extend through the dielectric layers in order to electrically interconnect the conductor layers as required. One method that has been suggested to facilitate the fabrication of multilayer circuit boards employs a radiation-sensitive dielectric coating material that is sequentially deposited to form permanent dielectric layers of a multilayer circuit, with vias that are photochemically patterned in the dielectric layers. These vias facilitate electrical connections between two or more conductor layers. A conductor pattern is then formed on the outer dielectric layer using conventional circuit board processes. As permanent dielectric layers of the circuit board, the coating material remains within the multilayer structure to electrically insulate adjacent conductor layers in all areas other than the via sites.
Another process for the manufacture of multilayered circuits entails sequential deposition of a plateable resin layer followed by a non-plateable resin layer. The two resins are preferably photo-definable, and the photo-definition of the two resin layers produces openings through which metal features of a circuit layer are formed. The first resin may include a catalytic filler that can be activated to enable electroless-plating of the resin with a metal, or may be surface catalyzed. The second resin does not contain a catalytic filler. The first and second resins are sequentially deposited and photochemically patterned, with the second resin overlying the first resin such that openings in the second resin expose regions of the first resin. These exposed areas of the first resin are then electrolessly plated to form metallic features such as pads, vias and conductors. In this manner, multiple circuit layers can be sequentially formed to produce a desired multilayer printed circuit board.
It has been suggested to fabricate a multilayer printed circuit board by depositing a first resin onto a metallically-patterned substrate. The first resin layer is then photodefined and a second resin layer is deposited on the first resin layer. The second resin layer is then photodefined. Openings in the first and second resin layers are developed by dissolving those portions of the resins that were not cross-linked during a patterning step. These exposed areas of the first resin are then electrolessly plated to form metallic features such as pads, vias and conductors. Alternatively, a laser can be used to drill the via openings through the deposited dielectric layers.
However, with any of the prior art methods, it is desired to increase the adhesion of the conductor and the resin layers to strengthen the multilayer circuit board. Therefore, a need exists for a method for forming a multilayer printed circuit board that provides enhanced interlayer adhesion strength.